Not Applicable
1. Field of the Invention
This invention pertains generally to nano-machining, and more specifically to nano-machining precise structures with a high ratio of thickness to feature size (also known as a high xe2x80x9caspect ratioxe2x80x9d).
2. Description of the Background Art
The need to machine structures of ever-decreasing dimensions is driven by many factors. One factor is that many materials at reduced dimensions exhibit unique physical properties that can be utilized effectively. An example is a quantum device. Another factor is that greatly improved performance can be obtained from precisely machined fine structures. For example, sensors and detectors with better sensitivity can be developed with structures of reduced dimensions. Furthermore, sufficiently miniaturized devices may have unique applications, such as medical implant and surgical devices. Additionally, the integration and dense packing of many miniature devices in one enclosure is often required to improve device performance in functionality or compactness.
Heretofore, several techniques have been used in the fabrication of precise structures with small features. Each technique has its own unique application with its associated limitations. For example, conventional electron or focused ion beam lithography is both capable of fabricating precise structures with feature sizes as small as 10 nm over a large area, but the aspect ratio of the nano-structures is limited to less than five due mostly to the difficulty of the transferring the pattern generated by electron beam writing machines to the desired material. X-ray lithography is used to make precise nano-structures with an improved aspect ratio compared to the e-beam or focused ion beam lithography but the small feature size is more limited. For an aspect ratio of ten, the achievable feature size is closer to 100 nm. The recently developed lithographic galvanoformung abformung (LIGA) technique is capable of producing microstructures with an aspect ratio as high as several hundred, but there the smallest feature size is limited to approximately 2 xcexcm.
One particular application requiring high aspect ratio nano-structures is the fabrication of zone plate lenses that comprise sets of concentric rings whose width decreases linearly with distance from the center. The rings are often made of alternating open slots and solid material. In fact, the rings do not need to be continuous, and the open slots can be solid material rings containing many holes and the solid material can be many dots.
Zone plates are among the most promising lenses being developed for x-ray applications. In the soft x-ray spectral region (roughly 0.1-1 keV), zone plates with the smallest zone width of approximately 20 nm and aspect ratios up to approximately five have been made using e-beam lithography and are being used for various imaging techniques. Zone plates have been used to produce images with a spatial resolution of approximately 25 nm. In fact, the spatial resolution is the best obtained in any imaging microscope using electromagnetic radiation, e.g., from extreme ultraviolet to visible light. Theory shows that the best resolution obtainable with a zone plate is equal to the outermost zone width, which is the smallest. Therefore, the ability to make precise zone plate structures with smaller zones thus improves directly the spatial resolution. However, the zone plate structure must have an adequate thickness in order to achieve an optimal focusing efficiency.
Although a sputtering/slicing technique which, in principle, is capable of producing high aspect ratio zone plates with spatial resolution approaching 10 nm, was proposed long ago, it has not reached its goal, and nearly all zone plates in use today with a spatial resolution better than 100 nm for soft x-ray (energy  less than 1 keV) are fabricated using various forms of an electron beam lithographic technique. For applications to x-ray energy greater than 1 keV, the aspect ratio required for optimal focusing efficiency increases with x-ray energy and the fabrication method successfully used for soft x-ray zone plates (electron bean lithography) can""t be directly used for producing zone plates of high resolution and high focusing efficiency for higher energy applications. Higher energy x-rays are often necessary for nondestructive imaging and examination of large objects or objects containing sufficiently large fraction of high atomic number elements. For example, a minimum x-ray energy of approximately 5 keV is required to have sufficient transmission through a 12-micron thick integrated circuit made of approximately 30% Cu and low atomic number elements. For 8 keV x-ray applications and assuming Au is the zone construction material, which yields close to the minimum aspect ratio required for a given focusing efficiency, a zone-plate needs to have an aspect ratio of approximately sixteen, forty eight, and one hundred sixty for the outermost zone width to be approximately 100 nm, 33 nm, and 10 nm, respectively. These required aspect ratios significantly exceeds the capabilities of the fabrication techniques hitherto, especially for the smaller outermost zone widths. The problem is worse for x-ray energies greater than 8 KeV.
Therefore, there is a need for a method for fabricating precise high aspect ratio nanometer structures. The present invention satisfies that need, as well as others, and overcomes limitations in conventional fabrication methods.
In general terms, the present invention is a method for xe2x80x9cnano-machining via particle-track-guided-etching of precise patternsxe2x80x9d. The invention combines (i) the precise nanostructure-patterning capability of electron beam lithography with (ii) the high aspect ratio nano-machining capability of a particle track etching method that employs a highly enhanced etching rate along particle tracks, which is analogous to machining by a drill bit of a nanometer sized diameter. More specifically, a lithographically generated pattern is used as a negative or positive mask defining etching areas, and the particle track etching method etches the unmasked areas along the directions guided by the particle tracks. The advantages of the invention for producing high aspect ratio nanostructures may also rest on the high mechanical strength of the wafer materials (e.g., insulators or semiconductors) compared to the organic resists used in conventional lithography techniques.
By way of example, and not of limitation, in accordance with the present invention a wafer, such as an insulator or semiconductor, is irradiated with a particle beam of suitable energy to break the chemical bonds of the material (e.g., radiation damage) and having a predetermined collimation at a desired direction with respect to the wafer surface. This step produces particle tracks for guiding the high aspect ratio nano-machining by particle track etching that will take place in a later processing step. Next, a thin layer of suitable patterning material such as e-beam resist is deposited on one side of the wafer, and an etch-stop layer of suitable material for preventing the etching of the particle tracks by the chemical solution used in a later step may be coated on the other side of the wafer. A desired pattern is then generated in the patterning material (e.g., e-beam resist) on the top surface of the irradiated wafer using conventional precise pattern generation techniques (e.g., electron or ion beam lithography), such that the patterning material is removed in selective areas thereby exposing the wafer surface. Removal of the patterning material would typically be carried out by etching via a chemical solution or reactive ion or other established technique. For example, the wafer would be placed in an appropriate chemical solution that etches along the direction(s) of the particle tracks over the areas that are not covered by the patterning material.
The etch rate along the particle tracks can be more than one-thousand times faster than the etch rate of materials not within the immediate vicinity of the particle tracks. The etched particle track area may have a diameter as small as 5 nm and a length as long as many thousand nm along the direction(s) of the particle tracks, depending on the wafer material, the charge, mass, and energy of the particles, and the etchant and the etching conditions. For example, the type and strength of the etching chemicals and etching time determines the length and diameter of the particle tracks for a given wafer material.
The structure in the area not covered by the patterning material after the etching process may contain isolated holes or closely connected holes thereby producing continuous lines, depending on the wafer material, particle density used in the irradiation process and etching conditions. The etched high aspect ratio precise structure in the wafer is one form of the final product. It can also be used for furthering processing such as a negative for molding via electroforming.
The present invention can be used to manufacture components for a wide range of applications, including miniaturized electromechanical devices and optical components. For example, it overcomes the main technical problems in fabricating high-resolution and high-focusing-efficiency zone plates and other optics for x-ray applications, namely, fabricating high aspect ratio nanostructures with precise zone plate pattern. This is achieved by using the process of the nanomachining via particle-track-guided-etching of precise patterns. Therefore, the invention disclosed here can be applied to produce zone plates with both high spatial resolution and focusing efficiency. For example, the invention can be used to fabricate zone plates with zone width as small as 5 nm with an aspect ratio up to one thousand and with a diameter several millimeters. Such zone plates can be used for many imaging and focusing applications for x-ray energies up to 70 keV with high spatial resolution and high focusing efficiency. Other x-ray optical components such as gratings and resolution test standards can also be made using the process of the nanomachining via particle-track-guided-etching of precise patterns. The invention can also be used as a machining method for removing gross areas of materials. Additionally, the invention can be used for machining three-dimensional products.
Further advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is provided for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.